ULTRAMICROSCOPY 71 



Very great care is thus necessary in the preparation of pure 

 water. Precipitation of suspended matter is often brought 

 about by adding a small quantity of a colloidal liquid, such as 

 ferric hydroxide ; the colloid is then precipitated (see below), 

 when the suspended matter is carried down with the colloidal 

 particles. Biltz filters through unglazed porcelain, filtration 

 through ordinary filter-paper, of course, not being permissible. 

 Malfitano has recently discovered that a film of collodion 

 serves very well for such purposes, and this is now commonly 

 employed. 



For a large amount of work, however, it is not necessary 

 (even if it were possible, which is doubtful) to remove all 

 traces of suspended matter. If the portion of the preparation 

 illuminated is in the form of a prism, with base 100 ^ square 

 and depth 5 fi, its volume would be 5 x 10 ~ 8 c.cm. Now, 

 recently distilled water contains about ten million particles in 

 a cubic centimetre ; therefore in the field of the microscope 

 there would probably be only one or two visible. Such a 

 degree of impurity will not affect the observations. Once such 

 liquids have been prepared, care must be taken that they are 

 not contaminated by the vessels into which they are placed. 



Experiments on the Rate of Chemical Action. — Many years 

 ago Tyndall performed some beautiful experiments on the 

 scattering of light by small particles. He allowed air to bubble 

 through nitrite of butyl, and then pass into an exhausted tube. 

 In addition, a little air which had passed through hydrochloric 

 acid was admitted into the chamber. A beam of light was 

 passed through the tube ; scattering occurred, the colour of the 

 scattered light changing from blue to white. A chemical action 

 takes place between the two compounds, a cloud being formed, 

 the drops which compose it gradually increasing in size ; this 

 increase can be followed by the change in the colour of the 

 diffracted light. 



Biltz has performed some similar experiments on the formation 

 of precipitates when optically pure aqueous solutions of certain 

 substances are mixed. Here, however, the growth of the 

 particles can be actually observed with the aid of the ultra- 

 microscope. When dilute sulphuric acid or oxalic acid is added 

 to a solution of sodium thiosulphate, after an interval, which 

 may be several minutes, the liquid becomes turbid as a result 

 of the precipitation of sulphur. An appreciable time elapses 



